Autoignition compositions

ABSTRACT

An autoignition composition  14  is provided to manage the safe combustion of a primary gas generant composition  12 . The autoignition composition  14  contains carboxylic acid and potassium chlorate. A method of managing the safe combustion of the primary gas generant composition  12  is also provided.

CROSS REFERENCE TO RELATED APPLICATION(S)

The present application claims the benefit of U.S. Provisional Application Ser. No. 60/606,322 filed on Aug. 31, 2004.

BACKGROUND OF THE PRESENT INVENTION

The present invention relates to vehicle occupant protection systems or gas generant systems in general. Various compositions employed in such systems include gas generant compositions, autoignition compositions, and booster compositions. Autoignition compositions, for example, are employed to auto-ignite at relatively lower temperatures than the main gas generant of the system in the event of a fire in the vehicle. Sequentially, the autoignition composition ignites to provide the impetus necessary to begin combustion of the main gas generant composition thereby preventing potentially harmful explosions. Certain pyrotechnic compositions are not necessarily suitable for autoignition compositions given that they tend to melt prior to their respective autoignition temperature thereby increasing the exposed composition surface area subject to combustion once the reaction begins. The result may therefore be a violent combustion reaction rather than a controlled combustion reaction. By controlling the effective combustion surface area of the autoignition composition once combustion begins, due to a fire around an associated vehicle for example, safer and more predictable management of the sequence of combustion reactions in a vehicle occupant protection system may be achieved.

Furthermore, pyrotechnic compositions such as autoignition compositions must also pass a thermal aging test at 107° C. for 400 hours to satisfy certain U.S. car safety requirements. At the same time, the post-aging autoignition temperature of the composition must not substantially deviate from the base-line or pre-aging autoignition temperature of the composition.

SUMMARY OF THE PRESENT INVENTION

The above-referenced concerns are resolved by the implementation of autoignition compositions within various gas generating systems, whereby the compositions are tailored to mitigate melting and a resultant increase in available combustion surface area, prior to ignition of the autoignition composition. An autoignition composition of the present invention includes a fuel formed from a racemic mixture of a carboxylic acid, or equimolar amounts of two optically active enantiomers of a given carboxylic acid. Accordingly, a first enantiomer may be characterized as a dextro-enantiomer indicating the optical activity of the enantiomer, and, a second enantiomer may be characterized as a levo-enantiomer indicating an optical activity exactly opposite of the first enantiomer. DL-tartaric acid is one example of a carboxylic acid of the present invention, whereby the fuel contains equimolar amounts of D-tartaric acid and L-tartaric acid.

A second component of the autoignition composition includes potassium chlorate as an oxidizer. The fuel and the oxidizer are mixed in respective amounts that are determined based on the design criteria of a given application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary airbag inflator containing an autoignition composition formed in accordance with the present invention.

FIG. 2 is a schematic representation of an exemplary gas generating system or vehicle occupant restraint system incorporating the inflator of FIG. 1 and an autoignition composition in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Autoignition compositions for vehicle occupant protection systems, for example, and more specifically, for airbag inflators, contain a mixture of potassium chlorate (KC), and a carboxylic acid. Carboxylic acid is selected from of DL-tartaric acid, succinic acid, glutamic acid, adipic acid, and mucic acid.

The choice of carboxylic acid is based on the desired autoignition temperature and degree of stability during thermal aging. For most automotive airbag inflator applications, it is desirable to have a composition that will survive aging for 400 h at 107° C. and will autoignite at a temperature of less than 200° C., or relatively close to 200° C.

EXAMPLES

Literature Amount Baseline Post-Aging Example Name MP ( C.) with KC Autoignition Autoignition 1 L-Tartaric Acid 168-170 43% 154 Ignited During Aging 2 D-Tartaric Acid 168-170 43% 154 Ignited During Aging 3 DL-Tartaric Acid 206 43% 185 188 4 Meso-Tartaric Acid 140 43% 146 Ignited During Aging 5 L-Tartaric Acid, >250  48% >250 >250  Mono-K Salt 6 DL-Malic Acid 131-133 35% >250 >250  7 Succinic Acid 188-190 30% 210 210 8 Diglycolic Acid 142-145 36% 155 213 9 Malonic Acid 135-137 39% >250 >250  10 Trans-Glutaconic 137-139 26% 188 211 Acid 11 D-Glutamic Acid 200-202 26% 235 230 12 Adipic Acid 152-154 22% 237 248 13 Mucic Acid 215 37% 223 221 14 2,2-Bis(hydroxymethyl) 189-191 23% >250 >250  propionic acid 15 Citric Acid 152-154 36% >250 >250  16 Phenylmalonic Acid 153 20% >250 >250  17 D-Quinic Acid 168 25% >250 >250 

Examples 1-17

Stoichiometric mixtures of each of the acids with potassium chlorate (KC) were made by grinding the raw materials separately in a vibratory grinder, and then blending by hand in a mortar and pestle. The autoignition test consisted of an aluminum fixture placed on a laboratory hot plate. The fixture was machined to accommodate a 0.3-0.5 g sample of autoignition material and a thermocouple probe. The tip of the probe was positioned directly below the autoignition composition, between the sample and the hotplate surface. The thermocouple was connected to a digital readout display and the hot plate was turned on to a heating rate of 30-50° C. per minute. The autoignition temperature was recorded as the temperature at which the sample combusted vigorously. Each sample was tested for thermal stability by placing about 15 g in a sealed glass vial and placing in a chamber at 107° C. for 400 hours. The hot plate autoignition was tested again after aging was complete.

Examples 18-19

Compositions were made containing DL-tartaric acid and KC in different ratios. This illustrates that the autoignition temperature does not substantially change if the composition is fuel-rich or oxidizer-rich. Auto- DL-Tartaric KC Oxygen Balance ignition Example Acid (wt. %) (wt. %) (wt. % O2) (C.) 18 30 70 +11.4 184 19 55 45 −11.7 183  3 (for comparison) 43 57 -0- 185

For comparison purposes, some of the compositions were tested on a TGA (thermogravimetric analyzer) with a smaller sample size and at a slower heating of 10° per minute. The results are as follows. This illustrates that the relative autoignition temperature does not substantially change with the sample size and heating rate. Composition From Hot Plate AI (0.3-0.5 TGA AI (20-50 Example g at 30-50 C./min) mg at 10 C./min) 1 154 142 3 185 171 13 223 200 11 235 213 12 237 222

Is should be noted that the stereochemistry of the carboxylic acids described herein is relevant to this invention. For example, Tartaric Acid has three stereoisomers as follows.

-   -   L-Tartaric Acid. Also known as L-(+)-Tartaric Acid or         (2R,3R)-(+)-tartaric Acid.     -   D-Tartaric Acid. Also known as D-(−)-Tartaric Acid or         (2S,3S)-(−)-Tartaric Acid.     -   Meso-Tartaric Acid. This is an achiral molecule describing the         (2R,3S) and (2S,3R) configurations.

L-Tartaric Acid and D-Tartaric Acid are enantiomers, meaning that they rotate the plane of polarized light an equal amount but in an opposite direction. DL-Tartaric Acid is a racemic mixture of L-Tartaric Acid and D-Tartaric Acid, meaning that it is an equimolar mixture of the enantiomers. As indicated in examples 1 and 2, L-Tartaric Acid and D-Tartaric Acid have the same melting point and function in the same way when mixed with KC. Meso-Tartaric Acid has different physical properties than both D-Tartaric and L-Tartaric Acid; it forms a hydrate and has a lower melting point, for example. When combined with KC, the autoignition temperature is lower, but it still ignites when aged at 107° C. As described in example 3, DL-Tartaric Acid has a higher melting point than either the D-tartaric Acid or L-Tartaric Acid alone, autoignites at a relatively higher temperature, and is stable when aged at 107° C. It is believed, although not confirmed, that the mixture of the two isomers is more stable because of hydrogen bonding. It will be appreciated that this concept may be applied to other carboxylic acids with more than one stereoisomer.

Carboxylic acid may be defined as given by Hawley, or as generally known by one of ordinary skill in the art. Hawley describes a carboxylic acid as any of a broad array of organic acids that primarily include alkyl (hydrocarbon) groups (CH₂, CH₃), usually in a straight chain (aliphatic), terminating in a carboxyl group (COOH). Exceptions to this structure are formic acid (HCOOH) and oxalic acid (HOOCCOOH). The number of carbon atoms ranges from one (formic) to 26 (cerotic), the carbon of the terminal group being counted as part of the chain. Carboxylic acids include the large and important class of fatty acids and may be either saturated or unsaturated. A few contain halogen atoms (chloracetic). There are also some natural aromatic carboxylic acids (b enzoic, salicylic) as well as alicyclic types (abietic, chaulmoogric).

The carboxylic acid(s) is/are provided at about 20-60 weight percent of the total autoignition composition, and the potassium chlorate is provided at about 40-80 weight percent of the total autoignition composition. The fuel and oxidizer are preferably dry mixed and if desired, compacted in a safe manner as known in the art. Exemplary carboxylic acids are listed as Examples 1-17 on the pages included herewith. All carboxylic acids are either provided by known suppliers or are manufactured by methods known in the art. For example, “Preparation of Carboxylic Acids” found at www.cem.msu.edu, is incorporated herein by reference. Furthermore, potassium chlorate may be supplied by Fisher or Aldrich Chemical, or any other known supplier. Or, potassium chlorate may be prepared by electrolyzing a hot concentrated alkaline solution of potassium chloride (mined from naturally occurring sites), or, by interaction of solutions of potassium chloride and sodium chlorate or calcium chlorate.

Racemization of a respective carboxylic acid may be achieved in a known manner. For example, U.S. Pat. No. 5,221,765, incorporated herein by reference, is instructional, and presents a known method of racemization of a carboxylic acid. An optically active carboxylic acid may, for example, be defined by the following formula:

where R₁, R₂, and R₃ are different and are hydrogen or C1 to C6 linear or branched alkyl, C1 to C6 linear or branched aralkyl, cycloalkyl, alkyl substituted cycloalkyl, C6 to C10 aryl, C1 to C6 linear or branched alkoxy, C6 to C10 aryloxy, C1 to C6 alkylthio, C2 to C8 cycloalkylthio, C6 to C10 arylthio, C6 to C10 arylcarbonyl, C4 to C8 cycloalkenyl, trifluoromethyl, halo, C4 to C5 heteroaryl, C10 to C14 aryl, or biphenyl unsubstituted or substituted with methyl or halo. The optically active carboxylic acid is heated in the presence of water at a temperature of from about 75 C to about 200 C in the presence of a catalytically effective amount of an aliphatic, aromatic, or mixed aliphatic and aromatic tertiary amine for a time sufficient to racemize the carboxylic acid. The time is typically about 4 to 24 hours depending on the amine employed and also depending on the carboxylic acid desired.

Other forms of carboxylic acids are also contemplated including tartaric acid, and those identified in the examples. It is believed that carboxylic acids have less ways to decompose than other fuels, and that the acidity destabilized the chlorate anion of the potassium chlorate oxidizer thereby facilitating fracture from the potassium. Compositions of the present invention are particularly useful as autoignition compositions for ammonium nitrate-based and phase stabilized ammonium nitrate-based primary gas generant compositions.

It is also believed that the present compositions provide a solid conflagration rather than a melt conflagration thereby ensuring a predictable surface area of combustion.

As shown in FIG. 1, an exemplary inflator incorporates a dual chamber design to tailor the force of deployment an associated airbag. In general, an inflator containing a primary gas generant 12 and an autoignition composition 14 formed as described herein, may be manufactured as known in the art. U.S. Pat. Nos. 6,422,601, 6,805,377, 6,659,500, 6,749,219, and 6,752,421 exemplify typical airbag inflator designs and are each incorporated herein by reference in their entirety.

Referring now to FIG. 2, the exemplary inflator 10 described above may also be incorporated into an airbag system 200. Airbag system 200 includes at least one airbag 202 and an inflator 10 containing a gas generant composition 12 in accordance with the present invention, coupled to airbag 202 so as to enable fluid communication with an interior of the airbag. Airbag system 200 may also include (or be in communication with) a crash event sensor 210. Crash event sensor 210 includes a known crash sensor algorithm that signals actuation of airbag system 200 via, for example, activation of airbag inflator 10 in the event of a collision.

Referring again to FIG. 2, airbag system 200 may also be incorporated into a broader, more comprehensive vehicle occupant restraint system 180 including additional elements such as a safety belt assembly 150. FIG. 2 shows a schematic diagram of one exemplary embodiment of such a restraint system. Safety belt assembly 150 includes a safety belt housing 152 and a safety belt 100 extending from housing 152. A safety belt retractor mechanism 154 (for example, a spring-loaded mechanism) may be coupled to an end portion of the belt. In addition, a safety belt pretensioner 156 containing propellant 12 and autoignition 14 may be coupled to belt retractor mechanism 154 to actuate the retractor mechanism in the event of a collision. Typical seat belt retractor mechanisms which may be used in conjunction with the safety belt embodiments of the present invention are described in U.S. Pat. Nos. 5,743,480, 5,553,803, 5,667,161, 5,451,008, 4,558,832 and 4,597,546, incorporated herein by reference. Illustrative examples of typical pretensioners with which the safety belt embodiments of the present invention may be combined are described in U.S. Pat. Nos. 6,505,790 and 6,419,177, incorporated herein by reference.

Safety belt assembly 150 may also include (or be in communication with) a crash event sensor 158 (for example, an inertia sensor or an accelerometer) including a known crash sensor algorithm that signals actuation of belt pretensioner 156 via, for example, activation of a pyrotechnic igniter (not shown) incorporated into the pretensioner. U.S. Pat. Nos. 6,505,790 and 6,419,177, previously incorporated herein by reference, provide illustrative examples of pretensioners actuated in such a manner.

It should be appreciated that safety belt assembly 150, airbag system 200, and more broadly, vehicle occupant protection system 180 exemplify but do not limit gas generating systems contemplated in accordance with the present invention.

In yet another aspect of the invention, a method of providing safe combustion of a primary gas generant in the event of a fire or high heat event is provided. By providing an autoignition composition 14 of the present invention in operable communication with the primary gas generant 12, the autoignition composition 14 provides a relatively gradual combustion profile for the primary gas generant 12. In the absence of an effective autoignition composition 14, the primary gas generant 12 might potentially melt prior to combustion thereby increasing the combustion surface area and reducing the predictability of the burn of the same.

Stated another way, a method of forming an inflator for a vehicle occupant protection system to safely combust a primary gas generant charge therein during a high-heat event such as a fire, is described as follows:

1. Provide an inflator within a vehicle occupant protection system, as known in the art.

2. Provide a primary gas generant composition suitable for use within the inflator.

3. Provide an autoignition composition in operable communication with the primary gas generant composition, whereby the autoignition temperature of the autoignition composition is less than the autoignition temperature of the primary gas generant composition, and, the autoignition composition contains a carboxylic acid and potassium chlorate. The carboxylic acid may be selected from one or more of the acids selected from the group of DL-tartaric acid, succinic acid, glutamic acid, adipic acid, and mucic acid. Or, the carboxylic acid may be also selected from one or more of the other acids described herein depending on the autoignition temperature desired. Accordingly, the acid may be selected wherein the ratio of the “melting point/post-aging autoignition temperature” ranges from about 0.80 to about 1.10, and, the ratio of the “pre-aging autoignition temperature/post-aging autoignition temperature” ranges from about 0.90 to about 1.20, and, the pre-aging autoignition temperature is about 50 C or more above the maximum heat aging temperature, 107 C at 400 hours typically.

Upon heating of the inflator, the autoignition temperature of the autoignition composition facilitates a managed combustion of the primary gas generant thereby resulting in a substantially solid conflagration with a predictable surface area, rather than a melt conflagration with a variable surface area and varied combustion conditions such as pressure within the pressure vessel or inflator.

Although the preferred embodiments of the instant invention have been disclosed in detail, it will be appreciated by one of ordinary skill in the art that the various aspects of the invention should not be limited to the embodiments described above, but should be given the full breadth as indicated in the claims appended hereto. For example, compositions herein disclosed are susceptible to modification without departing from the scope of the present invention, and additives known for their usefulness in vehicle occupant protection systems may be added to the present compositions for similar benefits attendant thereto. 

1. A method of managing the combustion of a primary gas generant within a gas generator of a vehicle occupant protection system, the method comprising the steps of: providing an inflator within a vehicle occupant protection system; providing a primary gas generant composition suitable for use within the inflator; and providing an autoignition composition in operable communication with the primary gas generant composition, whereby the autoignition temperature of the autoignition composition is less than the autoignition temperature of the primary gas generant composition and heterogeneous thereto, and, the autoignition composition contains carboxylic acid and potassium chlorate.
 2. The method of claim 1 wherein the carboxylic acid is selected from at least one acid selected from the group of DL-tartaric acid, succinic acid, glutamic acid, adipic acid, and mucic acid.
 3. The method of claim 1 wherein the carboxylic acid has a melting point/post-aging autoignition temperature ratio ranging from about 0.80 to about 1.10.
 4. The method of claim 1 wherein the carboxylic acid has a “pre-aging autoignition temperature/post-aging autoignition temperature” ratio ranging from about 0.90 to about 1.20.
 5. The method of claim 1 wherein the carboxylic acid has a pre-aging autoignition temperature about 50 C or more above the maximum heat aging temperature of 107 C.
 6. An autoignition composition for managing combustion of a primary gas generant composition in the event of a fire or heat event, said autoignition composition comprising: carboxylic acid selected from at least one acid selected from DL-tartaric acid, succinic acid, glutamic acid, adipic acid, and mucic acid; and potassium chlorate.
 7. The autoignition composition of claim 6 wherein said carboxylic acid is DL-tartaric acid.
 8. The autoignition composition of claim 6 wherein said carboxylic acid is provided at about 20-60 weight percent of the total composition, and, the potassium chlorate is provided at about 40-80 weight percent of the total composition.
 9. The autoignition composition of claim 6 wherein the carboxylic acid has a melting point/post-aging autoignition temperature ratio ranging from about 0.80 to about 1.10.
 10. The autoignition composition of claim 6 wherein the carboxylic acid has a “pre-aging autoignition temperature/post-aging autoignition temperature” ratio ranging from about 0.90 to about 1.20.
 11. The autoignition composition of claim 6 wherein the carboxylic acid has a pre-aging autoignition temperature about 50 C or more above the maximum heat aging temperature of 107 C.
 12. A vehicle occupant protection system incorporating the autoignition composition of claim
 6. 